He is very tall, compared to all the other characters. But you've got to take in consideration what Terra from KH: Birth By Sleep's height was, since they are practically the same. But Terra grew a lot in the time before getting a Nobody after Master Xehanort...yeah, i'd say he's pretty tall, but I cannot give an exact height because only the game creators know, probably. If anyone else knows, then they can edit my answer as long as they are nice about it, at least. Anyway, I didn't really answer your question, but at least someone else can tell you without having to look in the unanswered questions!
There is no possible way of finding Xemnas in kingdom hearts. You are only able of finding him in kingdom hearts 2, which he is the last boss in the game.
Xemnas
You kill xemnas with the help of riku
Xemnas in the world that never was
The scene that we see in Kingdom Hearts II Final Mix+ and the Opening in Kingdom Hearts 358/2 Days, where Xemnas gives Roxas a name, takes place right before Xemnas brings him into the Organization and introduces him to Axel. It's a flashback, so to speak.
In Kingdom Hearts 358/2 Days gameplay, Xemnas floats across the ground. This isn't to say that Xemnas doesn't walk at all; he just doesn't walk when you play as him in Mission Mode.
Fight and defeat Xemnas, the final survivor of Organization 13.
You riku Donal or Goofy Must defeat Xemnas and his forms!
Yes DIZ did destroy kingdom hearts but not Xemnas that's why Sora needed to destroy him for the second reason.
Xehanort, aka Xemnas, dies in kingdom hearts 2 when the player, teamed up with riku, defeats him in the final boss of the game.
You can load your last save and go back from there.
beat all the other cups and the first fight with xemnas.